Development of appressoria on conidial germ tubes of Erysiphe species

Appressoria-Small but Incredibly Powerful Structures in Plant-Pathogen Interactions

Plant-pathogenic fungi are responsible for many of the most severe crop diseases in the world and remain very challenging to control. Improving current protection strategies or designating new measures based on an overall understanding of molecular host-pathogen interaction mechanisms could be helpful for disease management. The attachment and penetration of the plant surface are the most important events among diverse plant-fungi interactions. Fungi evolved as small but incredibly powerful infection structure appressoria to facilitate attachment and penetration. Appressoria are indispensable for many diseases, such as rusts, powdery mildews, and blast diseases, as well as devastating oomycete diseases. Investigation into the formation of plant-pathogen appressoria contributes to improving the understanding of the molecular mechanisms of plant-pathogen interactions. Fungal host attachment is a vital step of fungal pathogenesis. Here, we review recent advances in the molecular mechanisms regulating the formation of appressoria. Additionally, some biocontrol agents were revealed to act on appressorium. The regulation of fungal adhesion during the infective process by acting on appressoria formation is expected to prevent the occurrence of crop disease caused by some pathogenic fungi.

Diversity and Function of Appressoria

Endophytic, saprobic, and pathogenic fungi have evolved elaborate strategies to obtain nutrients from plants. Among the diverse plant-fungi interactions, the most crucial event is the attachment and penetration of the plant surface. Appressoria, specialized infection structures, have been evolved to facilitate this purpose. In this review, we describe the diversity of these appressoria and classify them into two main groups: single-celled appressoria (proto-appressoria, hyaline appressoria, melanized (dark) appressoria) and compound appressoria. The ultrastructure of appressoria, their initiation, their formation, and their function in fungi are discussed. We reviewed the molecular mechanisms regulating the formation and function of appressoria, their strategies to evade host defenses, and the related genomics and transcriptomics. The current review provides a foundation for comprehensive studies regarding their evolution and diversity in different fungal groups.

Morphology and surface characteristics of the anamorphic stage of powdery mildew Erysiphe australiana on crape myrtle leaves

The morphology and surface characteristics of the powdery mildew Erysiphe australiana growing on crape myrtle leaves were observed with field emission scanning electron microscopy. The powdery mildew infection caused distortion and withering of the leaves, and nearly all external parts such as flowers, petioles, and branches were covered by the whitish colonies. Hyphal proliferation was prevalent on the adaxial surface of the powdery mildew-infected leaves. Globose ascocarp initials with hyphal aggregations were frequently seen on the leaf surface. Collapsed conidia showed longitudinal striations or ridges on the surface and deep linear wrinkling. Foot-cells were straight and grew at right angles from the vegetative hyphae. The conidiophores had fragmented, cylindrical, non-chained conidia which were produced singly at the apex of the conidiophores. The germ tubes formed intercalary multi-lobed appressoria and the conidia produced filiform protrusions emerging from subterminal positions. This study visualized previously unknown structures of E. australiana such as the ascocarp initials, filiform protrusions on conidia, and multi-lobed appressoria on germ tubes. These observations will facilitate the identification and taxonomy of this fungus and its allied species.